As paper or wood is inflammable, and metals have very low emissivity, ceramics have been widely used for far-infrared radiating material. Ceramics are nonflammable, and have higher emissivity.
Emissivity is expressed in watts per area (W/m.sup.2). Therefore, for materials of the same weight, the optimal condition for emissivity is satisfied with a material comprising particles having a larger specific surface area (m.sup.2 /g).
In the prior art, particles with an average particle size of not greater than 350 mesh, about 40 .mu.m, have been used. However, ultra-fine powder with an average particle size of not greater than 0.1 .mu.m, and a BET specific surface area of not less than 10 m.sup.2 /g, has rarely been prepared.
In the prior art, zirconia, alumina, or ceramics of transition metal oxides such as MnO.sub.2, Fe.sub.2 O.sub.3, CuO, which are similar to a black body, were preferably used for far-infrared radiating material. However, in some fields such as the food industry, the textile industry, etc, white color rather than black color is preferable for clearness of objects to which the material is applied.
U.S. Pat. No. 5,356,724 (Tsuda et al.) describes a material for producing far-infrared radiation comprising a substrate and an Al-Mn alloy-derived oxide surface layer on the substrate, the oxide surface layer being porous with an average pore diameter of 0.01-2.0 micrometers and a porosity of 10.sup.3 -10.sup.12 pores/cm.sup.2, the weight ratio of Mn/Al in the oxide surface layer being 0.001-2.0, and the thickness of the oxide surface layer being 0.1-100 micrometers.
U.S. Pat. No. 5,480,647 (Tsai) describes a medical compound comprising 70-90% by weight of powdered perlite and 10-30% by weight of at least one metallic oxide of which the metal is selected from the group of tantalum, niobium, palladium, nickel, germanium, titanium, and zirconium.
U.S. Pat. No. 5,028,760 (Shigeru et al) describes an infrared electric panel heater including an open side covered by a sintered or electroformed porous metallic panel. A sheathed electric heating element is installed in the housing in contact with the inner surface of the porous panel. The outer surface of the porous panel is substantially entirely covered with a ceramic layer, selected from the group consisting of Al.sub.2 O.sub.3, TiO.sub.2, Cr.sub.2 O.sub.3, MgO, ZrO.sub.2, SiO.sub.2 and mixtures thereof, which layer emits far-infrared rays when heated.
U.S. Pat. No. 5,058,196 (Nakamura et al.) describes a far-infrared heater having a plate-like porous metallic panel, formed by an electroforming process, installed in a box-like housing over an electric heating element therein. The panel is heated by the element and has its outer surface covered by a porous ceramic far-infrared emitting layer of Al.sub.2 O.sub.3, TiO.sub.2, Cr.sub.2 O.sub.3, MgO, ZrO.sub.2, SiO.sub.2 or mixtures thereof.
U.S. Pat. No. 5,542,194 (Hanzawa et al.) describes a body for emitting far-infrared radiation possessing high emissivity in a wide range corresponding to all the wavelengths of far-infrared radiation, which possesses excellent anticorrosivity and intensity properties. The body includes a ceramic containing SiC, preferably a Si-SiC ceramic.